This invention relates to moisture vapor barrier coatings made from selected modified starch esters and more particularly starch esters having an ester component of 2 to 8 carbon atoms and an intermediate DS of from 1.1 to 2.2.
In packaging and other applications a moisture barrier is sometimes needed to prevent moisture vapor transmission through the paper or other substrate material. In many cases, plastic materials such as polyethylene have been used to provide a moisture barrier or water resistant coating. While these materials are generally useful as protective coatings, they are not useful in paper product applications where repulpable and recyclable products are desired.
Starch based materials have been used in some coating applications. For example, U.S. Pat. No. 3,746,558 issued Jul. 17, 1973 to F. Berkhout et al. discloses an aqueous suspension of hydrophobic, low DS starch esters which can be used to provide water resistant coatings. U.S. Pat. No. 4,095,992 issued Jun. 20, 1978 to S. Rudolph et al. discloses mixed starch esters of mono- and polycarboxylic acid anhydrides as thermosetting, hydrophobic coatings that provide water resistance. European Patent No. 0 545 228 A1 published Jun. 9, 1993 shows the use of a mixture of modified starch and a synthetic polymer as a moisture barrier in moisture resistant sacks. Another European Patent No. 0 547 551 A1 published Jun. 23, 1993 discloses compositions comprising modified starch, gelatin, plasticizer, water and lipid as edible films which are effective in providing water, lipid, solute, gas, physical or microbial barriers in foods.
While the starch materials noted above do provide a moisture barrier and/or water resistance in different applications, they do not provide a thermoplastic coating material which can be readily processed to form a continuous film coating with good water vapor barrier properties.